Thermodynamic equilibrium feels comfortable because it satisfies our desire for order. For kinetic reasons, this ideal state is rarely established. Equilibrium describes the state of a system, but not how it got there. Still, we accept that metamorphic reactions will not occur at the P-T conditions predicted by the experimentally determined phase equilibria stability curves. Instead such reactions require metamorphic overstepping to provide the driving force to adjust the mineralogy to the new P-T conditions.